Bacterial pili are self-assembling filamentous protein appendages of the bacterial surface which grow out from the cell membrane into the cell environment. Some kinds of pili are virulence factors for bacteria causing infections of the urinary tract. Other kinds of pili are the infection organelle which transmit the genetic factors which confer resistance to antibiotics and the ability to produce certain toxins on bacteria. Pili may be considered as one component of a new class of viruses--epiviruses--which do not make a cell-free virion. Pili can also serve as organelles for bacteriophage entry, for DNA transformation competence, for the active movement of bacteria along a solid surface, and for specific adsorption of bacteria to other bacteria and to mammalian cells. This project is aimed at discovering, characterizing and classifying new pili, new pilus phages and new pilus functions; at elucidating the structure and function of the various kinds of pili; and at relating the fundamental properties of pili to the medical problems they cause. The techniques and disciplines of bacteriology, virology, genetics, biochemistry and biophysics are applied to these problems.